Bleed Air

BravoHotel

BravoHotel

Well-Known Member
I'm looking for clarification. Why is bleed air around 600 degrees F?

My instructor said it came from the turbine section of the engine (Allison T-56) as why it was so hot. Then I asked the question, does it go through a filtration system before it enters the AC system? I obviously got him thinking. So I started digging and it comes from the compressor section. Okay that makes more sense, you don't want by products of combustion circulating around your cabin. Does the temperature come from "Boil's Law" where when you compress a gas it heats up? But 600 degrees F seems very hot.

Am I correct in this theory? I just wanted some other input. Thankyou.
 
Hawks said:
I'm looking for clarification. Why is bleed air around 600 degrees F?

My instructor said it came from the turbine section of the engine (Allison T-56) as why it was so hot. Then I asked the question, does it go through a filtration system before it enters the AC system? I obviously got him thinking. So I started digging and it comes from the compressor section. Okay that makes more sense, you don't want by products of combustion circulating around your cabin. Does the temperature come from "Boil's Law" where when you compress a gas it heats up? But 600 degrees F seems very hot.

Am I correct in this theory? I just wanted some other input. Thankyou.
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The air is "bled" from part of the compressor section, not the turbine section. It is pre-combustion air, heated up because of the compression (and not because of the "fire"). Boyle's law has to do with pressure and volume...if my scratchy memory of Chemistry a dozen years ago holds, it's the ideal gas law that has to do with temperature and pressure remaining constant.

As far as what goes through the Environmental Control System of the interior of the aircraft, it all depends on the the systems in each individual aircraft. To turn bleed air into air conditioned air, the bleed air will go through any number of refrigeration processes before it comes out the vents into the cabin.
 
Boyle's Law is my guess too. The Ideal Gas Law says PV=nRT, so if you leave V (Volume), n (Number of moles of gas) and R (Ideal gas constant) alone (constant), then Pressure and Temperature are directly proportional. Boyle's Law says decrease the volume (force air into the compressor section) and pressure goes up. Pressure varies directly with temperature, thus temperature also goes up.

I wouldn't be so surprised by the high temp when looking at the opposite case, being when a CO2 canister or some kind of aerosol can is opened rapidly and gets frosty on the outside. If you've ever seen the scene in The Right Stuff when they're fueling the Bell X-1 with liquid oxygen and it's covered in ice and frost, or Apollo 13 when the Saturn V is covered in ice that's all shed during launch. This opposite case is called the Joule-Thompson Effect, and given how cold you can get stuff by expanding gas, I wouldn't be surprised you could reach 600 F by contracting it.

Who knows maybe there's also a little bit of thermal conduction or convection at work too being so close to the hot section of the engine (although it's probably pretty negligible).
(Disclaimer: I haven't taken thermo... yet. :) )

Hacker15e said:
As far as what goes through the Environmental Control System of the interior of the aircraft, it all depends on the the systems in each individual aircraft. To turn bleed air into air conditioned air, the bleed air will go through any number of refrigeration processes before it comes out the vents into the cabin.
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Exactly! Air conditioning "Packs" exist in pressurization/pneumatic systems because bleed air is too hot to be fed directly into the cabin.
 
Hawks said:
But 600 degrees F seems very hot.
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You'd be amazed. Our Rolls Royce engines pull bleed air from the 5th and 8th stages, which are only about two inches apart when you look at the engine. But the difference in those two inches is incredible. Air pulled from the 8th stage is a couple hundred degrees warmer than the 5th stage air. That journey of just a couple of inches further through the compressor does a hell of a lot.
 
As stated previously, each aircraft is different when it comes to cooling bleed air. That being said, the jets that I have flown have a "precooler" that mixes the tapped bleed air with fan air prior to entering any ducting. From there, it splits two ways:

1) It travels through an o-zone filter, primary and secondary heat exchanger (cooled by RAM air or a dorsal fan on the ground), and finally through a water separator (which is sometimes tapped back to further cool the heat exchangers).
2) It bypasses the cooling mechanisms (after the precooler).


The environmental control system then mixes the hot air (#2) with the cold air (#1) to the degree that the crew requests. Want it warmer? Select the knob to "hot" and more of #2 air will be mixed with the result of #1, etc.
 
Thank you everyone for the in-depth discussion. I basically understand how all the other systems work, anti ice, engine start, GTC/ATM, how the AC works, etc. The line was a little hazy around why the direct bleed air is used for the life support / pressurization systems.
 
Random fun fact - "pack" is actually an acronym for "pneumatic air conditioning kit."

OP, I have a Boeing 707 manual in SoCal that has a pack schematic, if you're really overly curious, I can have someone scan it, PDF it and send it to me to send to you.

inigo88 said:
I wouldn't be so surprised by the high temp when looking at the opposite case, being when a CO2 canister or some kind of aerosol can is opened rapidly and gets frosty on the outside. If you've ever seen the scene in The Right Stuff when they're fueling the Bell X-1 with liquid oxygen and it's covered in ice and frost, or Apollo 13 when the Saturn V is covered in ice that's all shed during launch. This opposite case is called the Joule-Thompson Effect, and given how cold you can get stuff by expanding gas, I wouldn't be surprised you could reach 600 F by contracting it.
Click to expand...
The J-T effect contributed to the loss of Thresher. Turns out that a failed dessication system plus non-full-bore air manifolds from high pressure air storage to the main ballast tanks = ball of ice when you do an emergency blow = no air to tanks = the little boat sunk.
 
Autothrust Blue said:
Random fun fact - "pack" is actually an acronym for "pneumatic air conditioning kit."

OP, I have a Boeing 707 manual in SoCal that has a pack schematic, if you're really overly curious, I can have someone scan it, PDF it and send it to me to send to you.
Click to expand...

There's a cool free schematic at this site for the Boeing 737 pneumatic system (including the PACKs). It also has photos of the water separator and compressor/turbine and primary & secondary heat exchangers. Pretty cool!

http://www.b737.org.uk/airconditioning.htm#Schematics

I still didn't quite get it until I read the basic wikipedia article on how an Air Cycle Machine actually works, and then it all made sense! You read the 737 schematic from the bottom up, starting with the pack valve, going through the primary heat exchanger allowing ram air to cool it a little bit (red to orange), through the compressor (which heats it back to red >185 C) and into the secondary heat exchanger which cools it down to blue (>99 C) and then forces it through the turbine, which not only keeps the compressor turning but allows the gas to expand on the other side and cool tremendously due to the J-T effect.

I've always felt pretty solid on systems but before this thread got us discussing (and got me dorking out reading and applying obscure ideas from chemistry and thermo) I always considered the contents of the packs to be some kind of freon-based wizardry. It's pretty amazing they can cool the air to -20 or -30 C just using RAM AIR, so by the end you have to mix some high temp bleed air back in just to get the intermediate temperature you want! Thanks guys, especially C150J for explaining most of this already.
This thread = what makes JC awesome. :beer:

Autothrust Blue said:
The J-T effect contributed to the loss of Thresher. Turns out that a failed dessication system plus non-full-bore air manifolds from high pressure air storage to the main ballast tanks = ball of ice when you do an emergency blow = no air to tanks = the little boat sunk.
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I wasn't familiar on the USS Thresher accident before your post. Thanks for the fascinating read. Not surprising that water separators were added to the air compressors after the disaster, although channeling a little secondary reactor plant steam around the valves/manifolds probably couldn't hurt either. "Emergency blow anti-ice?" ;)
 
inigo88 said:
I wasn't familiar on the USS Thresher accident before your post. Thanks for the fascinating read. Not surprising that water separators were added to the air compressors after the disaster, although channeling a little secondary reactor plant steam around the valves/manifolds probably couldn't hurt either. "Emergency blow anti-ice?" ;)
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The big change, mandated by SUBSAFE after the casualty, was that the lines from the HPACs to the air flasks, through the blow vales and into the tanks became "full bore" (same diameter throughout) with as few bends, kinks and valves as is practical. That way, the J-T effect is minimized, and even if the desiccant towers fail you have a better than average chance of getting high pressure air into the tanks.

There were a host of other changes ("save the mission, save the ship, save the plant, save the men") made too. Believe it or not, Submarines for Idiots has a good write up on that accident that contains the "inside" version of the accident. Along with a few other plain-English explanations of some nuclear reactor accidents (SL-1, for instance).

Also, I got your PM - will reply this afternoon.
 
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